1. Field of the Invention
The present invention relates to an off-track correcting method of a disk apparatus and a disk apparatus using the same, of measuring an off-track correction value by use of servo data of a data surface, this method being used for the disk apparatus for locating a head with a servo signal of a servo surface on a disk medium.
2. Description of the Related Art
A magnetic disk apparatus is requested to increase a storage capacity. A magnetic disk of the magnetic disk apparatus is formed with a servo surface to which a servo signal is written. Then, a data head on the data surface is on-track-controlled in accordance with the servo signal given from the servo surface.
Even in such a servo control system based on the above servo surface, if an environmental temperature changes, every magnetic disk is expanded and contracted to a different degree. Hence, the data head easily off-tracks even when executing the servo control. Prevention of this thermal off-track involves measuring an off-track correction quantity of the data head at an interval of a fixed time. Then, when modifying servo control, the data head is on-track-controlled by adding the off-track correction quantity. This is termed an "off-track correction".
In addition, it is requested that a track pitch be narrowed to increase the storage capacity of the magnetic disk apparatus. Therefore, it is required that the off-track correction quantity be precisely measured even when the track pitch is relatively narrow.
FIG. 12 is a diagram showing a construction in the prior art. FIG. 13 is a flowchart of an off-track correction value measuring process in the prior art. FIG. 14 is an explanatory diagram of the operation in the prior art.
As illustrated in FIG. 12, three sheets of magnetic disks 90 are rotated by spindle motor 91. In this example, five surfaces among six surfaces of the three magnetic disks are used as data surfaces 90-1, and one remaining surface is employed as a servo surface 90-2. The servo signal is written to the servo surface 90-2. This servo signal is defined as a 2-phase servo signal.
This data surface 90-1 is provided with a magnetic head (a data head) 92-1. The data head 92-1 reads and writes data from and to the data surface 90-1. Provided in an outermost position on the data surface is a servo cylinder recorded with the servo signal for measuring the off-track correction value.
Further, the servo surface 90 -2 is provided with a magnetic head (a servo head) 92-2. The servo head 92-2 reads servo data from the servo surface 90-2.
A rotary actuator (a voice coil motor) 93 moves these magnetic heads 92-1, 92-2 to be positioned in a radial direction of the magnetic disk 90. A servo demodulation circuit 94 generates two positional signals with a phase difference of 90 degrees from outputs of the servo head 92-2.
The data servo demodulation circuit 95, as mentioned below, demodulates five kinds of servo signals that are out of phase 90 degrees from the outputs of the data head 92-1. A read channel circuit 96 demodulates the data from the outputs of the data head 92-1. A control circuit 97 is formed with a processor.
A positioning operation of this magnetic disk apparatus will be described. The control circuit 97, upon receiving a seek instruction from a host device, executes coarse control. That is, a velocity curve corresponding to the number of tracks up to a target track is created. Then, a real velocity is calculated from a positional signal transmitted from a servo head 92-2. Obtained is a velocity error between the real velocity and a target velocity of the velocity curve. A rotary actuator 93 is controlled based on this velocity error.
A control circuit 97 detects a position from the positional signal. Then, upon detecting that a vicinity to the target is reached, the operation is switched over to fine control. More specifically, the control circuit 97 generates a fine control signal from the positional signal. Subsequently, the control circuit 97 controls the rotary actuator 93 on the basis of the fine control signal and an off-track correction value.
In the magnetic disk apparatus with the configuration mentioned above, the off-track correction value is measured and updated at the interval of a fixed time. For this measurement, the servo cylinder provided in an outermost position on a data surface 90-1 is formed with a servo pattern for an off-track measurement. That is, as illustrated in FIG. 14, the servo pattern for measuring an off-track correction value is formed extending from an "n-1" cylinder position to an "n+1" cylinder position. This servo pattern consists of five kinds of elements A, B, C, D, E.
A data servo demodulation circuit 95 demodulates detection outputs a, b, c, d, e of the respective elements A, B, C, D, E from outputs of the read head (a data head) 92-1. As illustrated in FIG. 14, when the read head 92-1 is located in a center position of the servo cylinder, the maximum output is the output c of the element C. When the read head 92-1 is located between the servo cylinder n and an inner position, i.e., the cylinder n-1, the maximum output is the output a of the element A. When the read head 92-1 is located in the inner cylinder position n-1 of the servo cylinder, the maximum output is the output e of the element E. When the read head 92-1 is located between the servo cylinder n and an outer cylinder position n+1, the maximum output is the output b of the element B. When the read head 92-1 is located in the outer cylinder position n+1 of the servo cylinder, the maximum output is the output d of the element D.
The off-track correction value has been, as shown in FIG. 13, measured by making use of the detection outputs described above.
To start with, the control circuit 97 drives the rotary actuator 93, and the servo head 92-2 is made to seek to the servo cylinder. At this time, a drive command value is obtained by adding the off-track correction value obtained by the measurement of the last time to a command value in the center position of the servo track. With this operation, the data head 92-1 is moved to the servo cylinder.
Next, the control circuit 97 checks whether or not a seek error occurs. Upon the off-track correction, it might happen that the servo head 92-2 is located in a boundary position of servo track on the servo surface. When located in this boundary, it follows that the servo detection output of the servo head 92-2 oscillates. As a result, the head oscillates enough not to converge at a predetermined position. A seek error due to this is detected.
When the seek error occurs, the control circuit 97 executes an error process. For example, the data head is inhibited from read/write operations.
On the other hand, if no seek error occurs, it follows that the servo head 92-2 and the data head 92-1 are located in that servo cylinder position. Then, the data head 92-1 reads the servo data in that position. For averaging, this operation is repeated three times.
An off-track correction value (an offset quantity) is obtained as follows from the detection outputs a, b, c, d, e of the respective elements A, B, C, D, E of the read servo data. When the data head 92-1 is located in a 3 .mu.m zone d1, the maximum output is the output a of the element A. When the output level of the element A comes to the maximum, an off-track correction value T is obtained from the outputs e, c of the elements E, C in the following formula: EQU T=e-c+3 (1)
When the data head 92-1 is located in a 3 .mu.m zone d2, the output b of the element B comes to the maximum. When the maximum output is the output b of the element B, the off-track correction value T is obtained from the outputs c, d of the elements C, D in the following formula: EQU T=c-d-3 (2)
When the data head 92-1 is located in a 3 .mu.m zone d3, the output c of the element C comes to the maximum. When the maximum output is the output c of the element C, the off-track correction value T is obtained from the outputs a, b of the elements A, B in the following formula: EQU T=a-b (3)
When the data head 92-1 is located in a 3 .mu.m zone d4, the output d of the element D comes to the maximum. When the maximum output is the output d of the element D, the off-track correction value T is obtained from the outputs b, a' of the elements B, A' in the following formula: EQU T=b-a'-6 (4)
When the data head 92-1 is located in a 3 .mu.m zone d5, the output e of the element E comes to the maximum. When the maximum output is the output e of the element E, the off-track correction value T is obtained from the outputs b', a of the elements B', A in the following formula: EQU T=b'-a+6 (5)
Note that the servo data is read three times, and the average value of the obtained off-track correction values is used as an off-track correction value to reduce a scatter in the off-track correction values as much as possible.
Thus, according to the prior art, there is measured the off-track correction value corresponding to the off-track quantity of the data head 92-1 when the data head 92-1 is located in the servo cylinder.
FIG. 15 is an explanatory diagram showing a problem inherent in the prior art.
As shown in FIG. 14, to be ideal, it is desirable that the off-track quantity and the off-track correction value be in a linear relationship. Referring to FIG. 14, the read head has 3.5 .mu.m for a track pitch 6 .mu.m.
As illustrated in FIG. 15, however, when a core width of the read head is as wide as, e.g., 4.5 .mu.m due to a manufacturing scatter, and when a characteristic deterioration of the head happens, as shown in FIG. 15, it is impossible for the off-track correction value to keep the linear relationship with respect to the off-track quantity. Particularly in disconnected points marked with circles in FIG. 15 between line segments, an unsensitive zone (a sensitivity deteriorated portion) might be produced as the case may be.
In such a phenomenon, an influence by the read characteristic of the read head becomes larger as the track pitch is narrower. Hence, when the data head is located in the sensitivity deteriorated position, there arises such a problem that the off-track correction quantity can not be accurately measured. This makes it difficult to accurately perform the off-track correction.